1. Field of the Invention
The present invention relates generally to methods for making dental restorations such as fillings, crowns, bridges, inlays, onlays, and veneers. The method involves curing a photopolymerizable composition in two separate steps. The composition can be referred to as a two-phase light curing (TPLC) material.
2. Brief Description of the Related Art
Dental professionals make and use different restorations depending upon the condition of the tooth and oral health of the patient. By the term, “restoration” as used herein, it is meant any product that replaces or restores lost tooth structure, teeth, or oral tissue including, but not limited to, fillings, inlays, onlays, veneers, crowns, bridges, splints, and dentures.
In dental anatomy, the outer (visible) portion of the tooth is referred to as the crown and it has a covering of enamel. The hard enamel protects softer dentin tissue in the upper portion of the tooth. The dentin tissue contains a matrix of minute tubules interspersed with collagen fibers that surround and protect the tooth pulp. The pulp includes connective tissue, blood vessels, cells, and nerve endings. The pulp cavity comprises an upper pulp chamber and root canals that extend to the apex or apical section of the tooth deeper into the jaw. The outer (non-visible) portion of the tooth root is covered with cementum, a thin hard tissue that joins the root to the surrounding bone. Dental decay, or caries, is caused by bacteria accumulating on teeth and forming a biofilm (plaque). The biofilm produces acids that dissolve and weaken the tooth, thereby causing decay.
When dental caries is found in the enamel portion, the dental professional will remove the caries to prevent further decay of the tooth. The dentist uses a high-speed handpiece equipped with a bur to “drill” the tooth. Then, the cavity is “filled” with a composite resinous material or silver amalgam filling. In recent years, composite fillings which can be made to closely match the shade of neighboring teeth have become more popular than amalgam fillings.
Composite filling material typically contains photopolymerizable resins such as an acrylate or methacrylate resin, reinforcing filler particles such as silica and glasses, polymerization initiators, polymerization accelerators, and various additives such as ultraviolet-light absorbers, anti-oxidants, plasticizers, and the like. The composite can be light-cured using a standard dental curing light (for example, SmartLite™ PS LED curing light, available from Dentsply) to form a hardened dental restoration. The curing light normally emits blue visible light in the spectrum of 400 to 500 nm to activate the polymerization initiator. The monomers undergo polymerization and the filling material hardens. In some instances, it is desirable to light-cure the composition in two successive steps. In the first light-curing step, the composition is partially-cured. The dental practitioner can work with the composition shaping and molding it as needed while in the partially-cured state. When ready, the practitioner light-cures the material a second time so that it fully cures and forms a hardened restoration.
For example, Schmitt et al., U.S. Pat. No. 5,472,991 discloses tooth filling compositions that are curable in two curing steps. The composition comprises polymerizable compounds such as monomeric and polymeric acrylates and methacrylates; filler materials such as silica particles; photoinitiator component I having a light absorption maximum of <450 nm such as bisacylphosphine oxides; and photoinitiator component II having a light absorption maximum of >450 nm such as camphorquinone. The composition is irradiated with light having a wavelength of >450 nm in a partial curing step to form a material having between 50 to 70% of its maxiumum hardness and then it is irradiated with light at a wavelength of <450 nm so that it completely cures.
As discussed above, a filling is placed in the tooth if the tooth structure has enough strength and integrity to hold the filling. A filling can be placed in different areas of the tooth, for example, the occlusal and/or mesial and/or distal portions and these fillings are rated as Class I to Class VI restorations depending upon the location and condition of the tooth. In some cases, however, the tooth structure has weakened considerably and it is unable to hold a filling. In these cases, a different restoration must be made for the tooth. For example, a dental crown can be placed over the tooth to restore the anatomy, function, and aesthetics of the natural tooth.
In conventional dental crown procedures, a patient must make multiple visits to the dentist. In the first visit, the dentist examines and prepares the tooth that will receive the crown. The dentist “preps” the tooth by filing and grinding it to a core. Next, the dentist takes an impression of the prepared tooth. The final impression is sent to a dental laboratory that will make the permanent crown. During the first office visit, the dentist places a temporary crown on the tooth to cover and protect it while the permanent crown is being fabricated. The temporary crown is made from a polymeric paste-like material such as an acrylic. At the second office visit, the dentist removes the temporary crown. The dentist cleans the tooth removing any residual temporary cement. Then, the color, margins, contacts, and occlusal fit of the permanent crown are checked. If satisfactory, the dentist affixes the permanent crown to the tooth using permanent dental cement.
This crown fabrication procedure, which involves making multiple visits to the dentist, can be time-consuming and costly. In recent years, there have been efforts to find new techniques whereby a dentist could design and fabricate the crown “chair-side” and mount the crown on the patient's tooth in a single office visit.
For example, Dias et al., US Patent Application Publication US 2007/0148623 discloses a method for making a dental crown in a single office visit. The method involves applying a spacer material to the patient's prepared tooth structure and then placing a transparent, plastic matrix containing the composite crown-forming material over the tooth. Because the matrix is transparent, light can be irradiated through the matrix and onto the composite material. This causes the material to polymerize and form a crown. The hardened crown is removed from the tooth; finished and polished; and then permanently mounted on the tooth using permanent dental cement. This chair-side technique for making a dental crown in a single office is a promising development. However, there is a need for an improved crown-forming material and method. The '623 Publication does not disclose a photopolymerizable composition containing two different photoinitiators and mixture of filler materials that could be cured in two distinct light-curing steps.
There is a need in the dental field for an improved restorative composition that can be cured in two separate and distinct light-curing steps. A dental practitioner should be capable of using the composition to make restorations having high mechanical strength and pleasing aesthetics. In addition, the composition should have good working time so that the dental practitioner can handle and place the material more effectively. The present invention provides such compositions having these benefits as wells as other advantageous properties. Methods of using the compositions to make chair-side dental crowns and other restorations such as chair-side inlays, onlays, veneers, and fillings are also included.